The present invention relates to the inspection of sheet material and, more particularly, to apparatus and method for inspecting sheet material having known characteristics.
Optical scanners have been used to inspect sheet material moving as a continuous web in a high-speed production line at rates up to several hundred meters per minute. The material can be, for example, paper, plastic, or metal. Such scanners may be set up to inspect the material for any number of characteristics measured by illuminating the material and comparing the intensity reflected or transmitted therefrom with a threshold model defined by already known or learned characteristics. The optical scanners include an array of photosites, each providing a pixel signal having a magnitude representing the intensity of light received from a corresponding point on the sheet material. Each photosite corresponds to the location of the point on the sheet material being inspected. The length of the array corresponds to the width of the sheet material so that the photosites provide successive sets of pixel signals, each set corresponding to a scan of the width of the sheet material.
As quality requirements have become more demanding, it has become desirable to inspect 100% of the sheet material rather than taking a sample. High-speed optical cameras are capable of providing pixel signals at rates up to 20 million per second. Thus, if an optical camera has a linear array of 1,000 photosites for scanning the width of the sheet material, it would provide about 20,000 scans per second which is more than sufficient to provide 100% sheet inspection on high-speed production lines. Although such cameras are currently available. economically-priced equipment for processing the pixel signals provided at the same high data speeds currently is unavailable.
Furthermore, even if such processing equipment were available, it is not necessary to process all of the pixel signals because only the deviations from the threshold model are important and typically represent only a small portion of the pixel signals being measured. It is more desirable to measure all of the pixel signals and process only those which represent a deviation from the threshold model, processing equipment would extract and store only that data which is significant, i.e., data indicating a deviation from the threshold model. Thus, it also becomes necessary to accurately define the threshold model to insure that most of the significant data is actually extracted from the stream of pixel signals being generated by the optical scanner.
Accordingly, there is a need for a method and apparatus for inspecting sheet material having known characteristics by comparing all of the pixel signals to a threshold model to accomplish 100% inspection of the sheet material, while extracting and storing only significant data from the stream of pixel signals and accurately defining and adjusting the threshold model to ensure that most of the significant data is extracted.